1. Field of the Invention
This invention relates to a lithographic printing plate developed by the silver complex diffusion transfer process. In particular, this invention relates to lithographic printing plates wherein an antihalation layer contains an acid ossein gelatin having a high bloom strength (i.e., high jelly strength) and/or low viscosity to increase the number of impressions the plate can produce.
2. Background of the Art
The present invention relates to a single sheet printing plate construction comprising a support, on which an antihalation layer, an emulsion layer and a physical nucleating receptor layer are provided. Following imagewise exposure, the plate is processed in a diffusion transfer (DTR) developer. The exposed area develops out as a black silver image, while the unexposed silver halide subsequently diffuses through to the receptor element. Physical development nuclei in the receptor layer reduces the silver halide and, in the presence of a toning agent such as 1-phenyl-5-mercaptotetrazole, forms an oleophilic silver complex.
As described in various embodiments, e.g., U.S. Pat. No. 3,728,114, U.S. Pat. No. 4,160,670 and U.S. Pat. No. 4,361,635, such a process can function as a lithographic printing plate due to the oleophilic nature of the complexed silver in the receptor layer and the hydrophilic developed silver image in the emulsion layer. With respect to the emulsion and antihalation layers, gelatin is the preferred medium, in particular, limed-treated inert ossein gelatin. To achieve acceptable abrasion resistance for running on a printing press, the gelatin has to be sufficiently hardened. However, even with excess quantities of hardener such as formaldehyde, the number of impressions this type of lithographic plate can make is limited. Furthermore, small amounts of hydroquinone developer that is incorporated into the printing plate to optimize diffusion transfer development interferes with the crosslinking process, thus reducing hardness and therefore lowering the run length.
U.S. Pat. No. 4,510,228 describes a diffusion transfer plate construction wherein all the gelatin containing layers have a pH below the isoelectronic point(pI) of the gelatin in each layer. Also, the preferred gelatin is time treated (column 3, lines 64-68).
U.S. Pat. No. 4,605,609 describes a low calcium/high bloom strength gel in the image receiving layer of a duplicating paper processed by the diffusion transfer process. However, it is well known in the art, and referenced in U.S. Pat. No. 3,728,114, that a hydrophilic colloid in the image receiving layer will inhibit the oleophilic function of the transferred image in DTR printing plate applications.
U.S. Pat. No. 4,606,985 claims a DTR printing plate wherein at least one gelatin layer contains between 3-30% of a low molecular weight gelatin. An especially preferred combination of gels in the underlayer of the plate construction is an acid treated gelatin and a low molecular weight gel (column 3, lines 50-54). However, no reference is made as to the viscosity of the acid treated gelatin, and that acid-treated gelatin was shown in examples to suffer from blinding and scumming in the absence of low molecular weight gelatin.
U.S. Pat. No. 4,743,525 describes a DTR printing plate having improved printing endurance, wherein the antihalation (AH) and emulsion layers contain a limed treated gelatin having a calcium concentration of 1,000 ppm or less. While the present invention uses gelatins having a calcium level of around 200 ppm, U.S. Pat No. 4,743,525 states that improved run length cannot be achieved with acid treated gelatins (column 2, lines 60-63).